- no 4G/LTE roaming
- in the UK often no fall-back to 2G
- Feel at home roaming is strictly restricted to 2 months per year. Using it for longer, it will be deactivated for roaming. This has been confirmed by many users.
- strictly no tethering (hotspot use) on PAYG SIM cards. They sniff and kill tethering connections.
- slow data due to high depriorisation and high latency when roaming
- UK content lock in place that affects even newsgroups
- no top-ups possible by a non-UK issued debit/credit card
- only ways are vouchers (sold in the UK) or top-up agencies with high (10-20%) fees

I don't really see any reason why anyone should use "feel at home" of Three UK in an EU country (and most of their destinations are EU/EEA) past June 15th, with the new EU roaming rules in place. I do acknowledge that there are some special cases left, when it may makes sense to use them like for the US, if you really don't need LTE and want T-Mobile and AT&T on one SIM.

And reading your post, honestly, it looks like spam for an eBay seller.
- you suggest to top-up the SIM by bank transfer???
- you suggest to continously use it for roaming???

Reading the eBay vendor's description, well, he knows where the issues are, but he keeps silent about them: It is recommended that you add credit when its convenient when you use this in UK. means you can't top it up from abroad (without high fees).You will need an unlocked 3G GSM phone. means no 2G in the UK and no 4G/LTE for roaming.
But he sells it for UK purposes mainly, so it may still be considered within regular ad talk and not lying.